Apparatus and method for determining thickness

ABSTRACT

Apparatus and method for measuring the thickness of a test piece by obtaining the acoustic velocity of the test piece where an ultrasonic wave is refracted into the test piece at a first point and the wave travels substantially parallel to the surface and the transit time to a second point where the wave is refracted out of the test piece is measured. The distance between the first and second points is known and the ultrasonic wave is initially transmitted into a member of different acoustic velocity than the test piece to produce the subsurface wave in the test piece.

This is a continuation of application Ser. No. 06/061,610, filed July30, 1979, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ultrasonic testing and more particularlyrelates to non-destructive ultrasonic testing technique for themeasurement of thickness of a test piece which includes determination ofthe velocity of ultrasonic energy in the test piece.

2. Discussion of the Relevant Art

There are many ultrasonic instruments which have been used for measuringthe thickness from one side of the material with the pulse-echo method.Such instruments require a calibration technique using test blocks ofsimilar material or by knowing the actual acoustic velocity of thematerial being measured, and then calibrating the instrumentcorrespondingly.

One method requires the use of two or more test samples having the sameor similar acoustic velocity as the material to be measured. These testsamples have known thicknesses and must cover the range of measurementexpected. The readout of the thickness measuring device is calibrated byfirst setting the zero control on the thinner of the two test pieces,and then setting the span control on the thicker of the two test pieces.

Another calibration technique uses the fact that the ultrasonicthickness measuring instrument can be calibrated in terms of acousticvelocity. If the acoustic velocity of the material being measured isknown, this information can be set into the instrument and only one testpiece is required to achieve a zero offset.

Material or structure such as pressure containment vessels without twoexposed and opposed measurable surfaces are not presently measurable forvelocity. The acoustic velocity is a function of the square root of theratio of the bulk elastic modulus of the test material to its density.In some materials, particularly cast iron, the acoustic velocity mayvary from sample to sample plus or minus 15% of a nominal value which isgenerally considered to be 2.0×10⁵ inches per second. If the actualacoustic velocity of the material under test is not known, thicknesstesting by the ultrasonic pulse echo method using a nominal value ofacoustic velocity may be quite erroneous.

Accordingly, the present invention provides a new and improved techniquefor determining the actual acoustic velocity of material to be testedwhere access to the material may be had from only one side thereof andthereafter measuring the thickness of the material.

Ultrasonic test equipment of the pulse-echo type generally may beconsidered to make thickness measurements using the velocity factor ofthe material as a base reference. Generally, in such equipment, the timeis measured between a transmitted pulse and the received echo. This maybe achieved by generating a rectangular waveform whose duration is ameasure of the transit time of the acoustic wave between opposedsurfaces. Knowing the transit time and the acoustic velocity in the testmaterial then leads to a simple arithmatic determination of thethickness of the material. Most ultrasonic thickness gauging instrumentsoperate on this theory. Typical instruments are the DIGI-SONIC 502manufactured by Sonic Instruments, Inc. of Trenton, N.J. and also aModel 220, manufactured by the same company. These instruments areportable, designed for field testing, and at least one will be partiallydescribed in conjunction with the invention.

SUMMARY OF THE INVENTION

Briefly stated, the invention in, one form thereof comprises apparatusand the steps of positioning a first ultrasonic transducer at an angleto the surface of the test piece such that the angle of refraction ofthe ultrasonic wave entering the test piece is near a first criticalangle and creates a longitudinal subsurface wave in the test piece;positioning a second receiving transducer at the same distance and atthe same angle from the test piece so that the axes of the transducersintersect the test pieces to define a known dimension. Then, the firsttransducer is excited and an ultrasonic wave entering the test piece isrefracted to be a longitudinal subsurface acoustic wave. As this wavepropagates along the test piece it decays, emitting energy which isrefracted back to the second transducer where the second transducer axisintersects the test piece. The time of propagation is thus measured andsince the distance between the intersecting axes is known, the acousticvelocity in the test piece is determined. When the acoustic velocity inthe test piece is determined, the instrument may then be calibrated forthis velocity, and the thickness of the test piece determined by theknown pulse echo technique.

An object of this invention is to provide a new and improved method andapparatus for determining the acoustic velocity of a test piece and thethickness thereof.

BRIEF DESCRIPTION OF THE DRAWING

In order that the invention may be more fully understood, it will now bedescribed by way of example, with reference to the accompanying drawingin which:

FIG. 1 is a simplified block diagram of an ultrasonic measuring systemin which the principles of the present invention may be utilized;

FIG. 2 is pictorial representation of a piece of material having onesurface available for a thickness measurement with the ultrasonicmeasuring transducers coupled to said surface; and

FIG. 3 is a typical timing wave form diagram of the system disclosed inFIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An ultrasonic measurement system 10 in which the invention may beutilized is exemplified in block form in FIG. 1 and is shown in atwo-transducer mode of operation. System 10 comprises a transmitter 11adapted to excite a first ultrasonic transducer 12, and a receiver 13adapted to receive a reflected or transmitted ultrasonic wave from asecond transducer 14. The transmitted pulse as well as a received pulseare applied to a pulse duration generator 15 which will generate a pulseproportional to the time between the transmitted pulse and the receivedsignal. The output of the pulse duration generator is applied to a pulseduration to voltage converter 16 which generates a voltage proportionalin magnitude to pulse duration. The output of voltage converter 16 isapplied to an analog to digital converter 17 which will supply a binarycoded decimal signal to a digital display 18, which is conventionally ofthe seven bar character type. The system also includes an arithmaticallogic network 19 in which various intelligence may be set into theinstrument and the instrument calibrated. For example, in the modeselection, the operator will set the unit either to read out distance orvelocity. When distance is to be measured, the known velocity of thematerial under test will be set into the instrument. Also provided is acalibration adjustment which permits among other things certain pulsesto be ignored or measurement selected in accordance with pulsesoccurring on a known relative time basis. The instrument as shown is inthe dual transducer mode but for straight thickness testing may beutilized in a single transducer mode; that is, in the pulse echo mode ofoperation. An instrument generally as shown is marketed by SonicInstruments, Inc. of Trenton, N.J. under the designation DIGI-SONIC 502.

FIG. 2 exemplifies a piece of test material T to which there is onlyupper surface S availability. An adapter member referred to as a wedge20 is acoustically coupled to the upper surface S of the piece T by acoupling fluid. The wedge 20 may be of a plastic material known asmethyl methacrylate or polystyrene, or any other homogeneous materialhaving lower acoustic velocity than test piece T. The wedge is formedwith spaced apart surfaces 21 and 22 which are disposed at predeterminedangles. This angle is selected to provide a complementary angle A of theaxes of transducers 12 and 14 with the vertical. The angle A is close toa first critical angle and is so chosen that upon refraction of theultrasonic wave at the surface S it will produce a longitudinalsubsurface wave generally indicated by the envelope E. This wave willradiate and decay as it propagates in the test piece T. At a point Pthere will be energy refracted back into wedge 20 at the same anglewhich will be received by transducer 14. The distance between the pointP and P1 where the ultrasonic energy is first refracted into the testpiece T is a known dimension as is the distance from the transducer 12to point P1 and point P to transducer 14 on the transducer axes. Thedistance between P1 and P is the distance between the points ofintersection of the axes 12a and 14a of transducers 12 and 14,respectively, with the bottom of wedge 20. This is a known distance L.The distance D between transducer 12 and point P1, and transducer 14 andpoint P are known to be the distance D which is preferably equal at eachend of wedge 20.

The angle A between the normal to the test piece T and the axes of thetransducers is selected to be close to the first critical angle ofdiffraction of the ultrasonic wave into the test piece; that is, uponstriking the test piece T, the ultrasonic wave from transducer 12 willcreate a subsurface wave substantially parallel to the surface S of testpiece T, and propagate generally as shown by the envelope E.

The acoustic velocity of materials vary dependent on the bulk elasticmodulus and the density of the material. Thus, as the ultrasonic energyor wave moves from one material to another there is a change in velocityof propagation and hence refraction of the energy at the interface ofthe different materials.

This phenomena is similar to the refraction of light at a lens surface.There is an angle of criticality beyond which the wave does not travelinto the material, but will become a subsurface wave substantiallyparallel to the surface.

The critical angle A may be calculated from Snell's law considering theangle B to be 90° and using the nominal value of acoustic velocity for agiven material as follows: ##EQU1## where V1 is the longitudinalacoustic velocity of the material of the wedge,

V2 is the longitudinal nominal acoustic velocity of the material undertest,

A is the angle of the ultrasonic energy in the wedge with respect to thevertical, and

B is the refracted angle in the material under test.

Since B is 90°, Sin B=1.

Therefore, ##EQU2##

Where the wedge is a methyl methacrylate (Lucite) typical angles A are

    ______________________________________                                        Material     Nominal Velocity                                                                             Angle A                                           ______________________________________                                        Aluminum     2.5 × 10.sup.5 inches/sec.                                                             25°                                        Steel        2.3 × 10.sup.5 inches/sec.                                                             27°                                        Cast Iron    1.8 × 10.sup.5 inches/sec.                                                             35°                                        ______________________________________                                    

The velocity in methyl methacrylate of the type identified by thetrademark Lucite is 1.05 by 10⁵ inches/second.

The velocity in the wedge will determine the angle A. Where the wedge ispolystyrene, the angle A is as follows for various materials:

    ______________________________________                                        Angle A     Material and Velocity                                             ______________________________________                                        22°  Aluminum  2.4 × 10.sup.5 inches/sec.                        24°  Steel     2.3 × 10.sup.5 inches/sec.                        28°  Cast Iron 2.0 × 10.sup.5 inches/sec.                        ______________________________________                                    

The velocity in the wedge must be different than that in the test piecein order to obtain a critical angle and produce a subsurface wave.

FIG. 3 is exemplary of timing waveforms in the system of FIG. 1. At timet₁, a clock signal C goes high energizing transducer 12 which producesan ultrasonic wave in the wedge. At time t₄, the transmitted andrefracted wave is detected by transducer 14. The interval between timest₁ and t₄ is the total transit time of the ultrasonic energy in thewedge 20 and the test piece T. Since the velocity of the wave in thewedge V₁ is known as is the distance D, the instrument may be calibratedto ignore the intervals t₁ to t₂ and t₃ to t₄ in the velocity readout.

In any event, the acoustic velocity V₂ in the test piece may becalculated from a time basis as follows:

    t.sub.4 -t.sub.1 =(t.sub.2 -t.sub.1)+(t.sub.3 -t.sub.2)+(t.sub.4 -t.sub.3)

and ##EQU3## where V₂ is the acoustic velocity in the test piece 20.##EQU4## since 2D/V₁ =(t₂ -t₁)+(t₄ -t₃) ##EQU5##

Ultrasonic waves can be propagated to some extent in any elasticmaterial. This traveling of sound waves occurs as a displacement of thesuccessive elements of the medium. In any elastic medium, there is arestoring force which tends to restore each element of material back toits original position after movement. Since all elastic substances alsopossess inertia, the particle continues to move after it returns to thelocation from which it started, and finally reaches another locationpast the original one. It will then continue to bounce back and forthwith constantly diminishing amplitude. The particles of the materialwill execute different movements, or orbits, as the wave passes throughthem. The overall effect is to attenuate the strength of the ultrasonicenergy traveling through this medium.

Longitudinal, or compression, waves exist when the motions of theparticles of a medium are parallel to the direction of the waves. It isthe type used when employing the straight beam technique of testing.This wave is most often used in ultrasonics since it will travel inliquids or solids, and is easily generated and detected. Longitudinalwaves have a high velocity of travel in most materials, and thewavelengths in common materials are usually very short in comparisonwith the cross-sectional area of the crystal used. This property allowsthe ultrasonic energy to be directed into a sharp beam.

When shear waves are generated in a material, the movements of theparticles in the medium are at right angles to the direction of wavepropagation. They usually travel in a beam of small cross-section. Shearwaves have a velocity that is approximately half of that of longitudinalwaves. The shear wave is the type that is generated when using an anglebeam technique of testing for defects.

The velocities and angles given supra are for generation of alongitudinal wave as opposed to a shear wave.

The angle A may be exceeded over the values given up to a point wherethe received output is substantially reduced. Since the longitudinalwave is used to measure thickness, it is preferred to use thelongitudinal wave for velocity measurement. Therefore, the angle A ispreferably not increased over the first critical angle to an angle wherea subsurface shear wave is generated substantially parallel to thesurface. In producing the longitudinal subsurface wave at or above thefirst critical angle A, there will also be a refracted shear wave in thetest piece. However, the refracted shear wave will not becomesubstantially parallel to the surface until the second critical angle isreached.

The wedge is preferably constructed so that once the acoustic velocityof the test piece is known the thickness of the test piece may bequickly measured. The height of the wedge is the same dimension as thedistance D. Since the instrument is still compensated for the distance Din the wedge, the instrument may be set to the thickness mode and thethickness W of the test piece measured as shown using the transducers12' and 14' on top of the wedge. Alternately, the thickness may bemeasured using only one transducer in the pulse echo mode.

In the thickness mode the dimension W is ##EQU6## where t_(o) is theround trip transit time in test piece T and V₂ has previously beendetermined. The bottom surface of the wedge 20 may be contoured inaccordance with the surface of the test piece.

Where the instrument used is of the pulse echo type; that is, the timebetween pulse and echo is a measure of the round trip transit time andthe arithmatical logic is designed accordingly, a factor of two must beintroduced for measuring between the transmitted pulse and receivedpulse by separate transducers.

It may thus be seen that the objects of the invention set forth as wellas those made apparent from the foregoing description are efficientlyattained. While a preferred embodiment of the invention has been setforth for purposes of disclosure, modification to the disclosedembodiments of the invention as well as other embodiments thereof mayoccur to those skilled in the art. Accordingly, the appended claims areintended to cover all embodiments of the invention and modifications tothe disclosed embodiment which do not depart from the spirit and scopeof the invention.

What is claimed is:
 1. A method of measuring the thickness of a testpiece with sonic energy, comprising the steps of:(A) providing a memberof different material than said test piece, said member having;(i)spaced apart angular surfaces approaching an angle complementary to thefirst critical angle of refraction of said test piece, each said angularsurface adapted to receive a transducer, (ii) an upper surface disposedbetween said angular surfaces and (iii) a bottom surface; (B)acoustically coupling said member to the surface of said test piece; (C)positioning a first transducer upon one of said test piece angularsurfaces such that the critical angle of refraction of said test pieceis approached; (D) positioning a second transducer upon the other ofsaid test piece angular surfaces at the same critical angle from saidtest piece surface so that the axes of said first and second transducersintersect said test piece surface to define a known dimension along saidtest piece surface; (E) exciting said first transducer to provide asonic subsurface wave in said test piece substantially parallel to saidtest piece surface; (F) measuring the time of transmission of said sonicwave between said first and said second transducers; (G) calculating thevelocity of said sonic wave in said test piece; (H) positioning a thirdtransducer on said member upper surface, the distance between saidmember upper surface and said member bottom surface being equal to thedistance of said first transducer, measured along the axis of said firsttransducer, to the intersection of said first transducer axis with saidmember bottom surface; (I) exciting said third transducer to transmit anultrasonic pulse in said member and said test piece; (J) detecting anecho pulse from the bottom side of said test piece; and (K) measuringthe thickness of said test piece by determining the round trip transittime of a sonic pulse and echo in said test piece.
 2. The methodaccording to claim 1 further including a fourth transducer disposed onsaid member upper surface adapted to receive said echo pulse used formeasuring said test piece thickness.
 3. An apparatus for measuring thethickness of a test piece, comprising:(a) a member having;(i) spacedapart first and second angled surfaces arranged to receive first andsecond ultrasonic transducers, said angular surfaces being at equalangles approaching an angle complementary to the first critical angle ofrefraction of said test piece, (ii) an upper surface disposed betweensaid angular surfaces, and (iii) a bottom surface; (b) first, second andthird ultrasonic transducers, said first and said second transducersbeing disposed on said first and said second angular surfaces,respectively, having axes which intersect the bottom surface of saidmember to define a predetermined dimension along said test piecesurface, said third transducer being disposed on said member uppersurface, the distances of each said transducer to said member bottomsurface, as measured along each said transducer axis being the same, theaxes of said first and said second transducers defining an angle withthe normal to said member bottom surface approaching a first criticalangle of diffraction of the ultrasonic energy between said member andsaid test piece; (c) first means for calculating the velocity of a sonicsubsurface wave traveling substantially parallel to said test piecesurface generated by said first transducer and received by said secondtransducer; and (d) second means for calculating the thickness of saidtest piece by measuring the round trip transit time of an ultrasonicpulse generated by said third transducer and the acoustic echo pulsereturning from the bottom side of said test piece and received by saidthird transducer.
 4. The device of claim 3 where the bottom surface ofsaid member is contoured to be complementary to a given test piece. 5.An apparatus according to claim 3 further including a fourth transducerdisposed upon said member upper surface, said fourth transducerreceiving said echo pulse.
 6. An apparatus according to claim 3 furtherincluding means for displaying said thickness measurement.
 7. In anapparatus for measuring with ultrasonic energy the thickness of amaterial under test having a means for calculating the thickness of saidmaterial by utilizing a first transducer to determine the transit timeof an ultrasonic pulse and echo in said test piece and displaying saidinformation, the improvement of obtaining the acoustic velocity in saidtest piece, comprising the steps of:(a) positioning a second transducerat an angle to one surface of said test piece such that the criticalangle of refraction is approached; (b) positioning a third transducer atthe same angle from said test piece surface so that the axes of saidsecond and third transducers intersect said test piece surface to definea known dimension along said surface; (c) exciting said secondtransducer to produce a sonic subsurface wave in said test piecesubstantially parallel to said test piece surface; (d) measuring thetime of transmission of said sonic wave between said second and saidthird transducers; (e) calculating the velocity of said sonic wave insaid test piece; and (f) positioning said first, said second and saidthird transducers so that the distance from each transducer, measuredalong its respective axis, to said test piece surface is the same.
 8. Anapparatus according to claim 7 further including the step of:(g)providing a fourth transducer disposed the same distance, measured alongits axis, from said test piece surface as said first, said second andsaid third transducers, said fourth transducer being used to receivesaid echo caused by exciting said first transducer.